The present invention relates generally to the field of exploration and production of oil and other fossil fuels from a well, and more particularly, to a pressure accumulator apparatus, system and method for use in offshore drilling and production.
In order to extract natural resources from reservoirs beneath the floor of large bodies of water, drilling operations have been conducted offshore. In a typical offshore drilling system, a platform floating in the ocean or installed on the seabed is used to support an offshore drilling rig. A riser string is typically provided between the platform and the wellhead at the ocean floor.
Due to the continual demand for precious natural resources, the oil industry has continued to push the water depth frontiers of exploratory drilling and production of oil and gas in subsea environments. As a result of the venture into deeper waters, many challenges have arisen. Indeed, safety and other considerations in offshore drilling operations require that the drilling equipment be able to rapidly close the well bore in order to prevent accidental blowouts, regardless of the water depth.
Many drilling platforms include high pressure valve systems known as blowout preventers (BOPs) as a standard part of the drilling equipment. BOPs are utilized to quickly shut off the uncontrolled flow of pressurized fluids. A BOP typically involves delivery of pressurized hydraulic fluid into and out of opening and closing chambers to control force urging seal elements into sealing relation with drill pipe, tubing or a wireline extending through a casing, or to move seal elements into sealing relation with each other to close off an open hole. Several different types of BOPs may be stacked together to provide control under various circumstances.
Conventional hydraulic BOP control systems experience delays in operating subsea BOP functions in deep water applications because the time required to send a hydraulic activation signal through an umbilical hose from the surface control station to the subsea pilot control valve becomes excessively long in deep water. Furthermore, delivery of sufficient quantities of pressurized operating fluid to the BOP from the surface requires a substantial amount of time.
Therefore, in lieu of transferring hydraulic fluid or hydraulic control pressure from the surface of the drilling rig through a hydraulic conduit on the riser, the hydraulic fluid can be transferred to accumulators. Accumulators are generally known as vessels which contain hydraulic fluid under pressure for use in effecting BOP closure. Through the use of compressed gas such as nitrogen, accumulators store energy which can be used to effect rapid BOP closure. Various types of accumulators have been used in the prior art, such as bladder types and guided float types, some of which actuate a spring-loaded liquid inlet-outlet valve.
As the water depth varies, the pressure of nitrogen gas that is in the accumulator varies to balance the effect of the hydrostatic pressure needed to maintain the efficiency of hydraulics. As the control system is lowered through the ocean water, the hydrostatic pressure increases, and the hydraulic stability within the accumulator is lost because it does not have an accurate pressure reference that is of the correct pressure. Uncertainty therefore arises as to whether the device is functioning properly as it travels down to the well head. Furthermore, the number of accumulators needed to provide useable fluid to operate the BOP may be difficult to ascertain. In addition, the number of accumulators that may be used is limited by the size of the BOP frame.
Furthermore, conventional bladder-type systems in the prior art are constrained in that as the system is lowered into the ocean water, multiple stops may be required depending on the water depth in order to recharge the system, due to a loss of useable hydraulic fluid as the water depth increases. To stop the operations of lowering the BOP stack through the water requires connecting a hydraulic line, flushing the line so that salt water does not enter the control system, and then recharging the accumulators. Typically, this process can take a significant amount of time. As the costs of operating an offshore rig often approach several thousand dollars per hour, frequent and lengthy delays in operations therefore contribute to substantial increases in the costs of the drilling operations.
Further difficulties arise with regard to performing preventive maintenance on the equipment. The equipment is very comprehensive in terms of component count, system architecture, and complexity. The accumulators must be removed to reach the hydraulic equipment, and then later reconnected. This is very burdensome and time-consuming, and creates concern for single-point failure within the hydraulic system.
A need has therefore arisen for an apparatus, system, and method that overcomes the limitations of the prior art. A reduction in the required number of accumulators would improve the accessibility to other critical components and reduce the amount of time required for equipment maintenance. The elimination of the need for having high-pressure booster pumps and banks of nitrogen bottles located on the rig, which expose personnel to safety hazards, would be an improvement upon the prior art. The foregoing improvements would greatly facilitate offshore oil production at greater depths of water while reducing operation and equipment costs as well as safety hazards.
Accordingly, the present invention provides an improved pressure accumulator device for use in deep offshore drilling operations. In accordance with a preferred embodiment of the present invention, a pressure accumulator apparatus for use in offshore drilling and production comprises a pressure vessel having a first compartment and a second compartment, a partition between the first compartment and the second compartment, a means for admitting fluid at ambient pressure into a portion of the first compartment, a means for controlling an amount of hydraulic fluid ejected from a portion of the second compartment, a first piston circumscribed by an interior wall of the first compartment, said first piston being capable of sliding within the first compartment, a second piston circumscribed by an interior wall of the second compartment, said second piston being capable of sliding within the second compartment, and a means for connecting the first piston and the second piston. During deployment, fluid, i.e., water, at ambient pressure enters the pressure accumulator device. As the BOP system is deployed and the hydrostatic pressure from the water increases, the piston will cause a balancing effect on the hydraulic fluid stored in the pressure accumulator device. Thus, the piston rod will either be in tension or compression.
Also disclosed is a system for delivering pressurized hydraulic fluid to a BOP for use in offshore drilling and production comprising a plurality of pressure accumulator devices in accordance with a preferred embodiment of the present invention, a means for serially connecting hydraulic fluid volumes of the accumulator devices, and a structure on which to mount the plurality of dual piston pressure accumulators.
A preferred method of pressurizing hydraulic fluid in a dual piston pressure accumulator for use in offshore drilling and production is also disclosed, comprising the steps of drawing a vacuum in the volume of a first portion of a first compartment of a pressure vessel, said first portion being adjacent to a first side of a first piston circumscribed by the first compartment of the pressure vessel. Said method further comprises the step of injecting a gas to pressurize, to a specified pressure, the volume of a first portion of a second compartment of the pressure vessel, said first portion being adjacent to a first side of a second piston circumscribed by the second compartment of the pressure vessel, wherein the second piston is connected to the first piston. Said method further comprises the steps of filling with hydraulic fluid the volume of a second portion of the second compartment of the pressure vessel, said second portion being adjacent to a second side of the second piston, and admitting ambient fluid into a second portion of the first compartment of the pressure vessel, said second portion being adjacent to a second side of the first piston member.
The gas typically used is nitrogen gas (N2). An object of the present invention is to provide a pressure accumulator that eliminates the need for high precharges of nitrogen which expose personnel to safety hazards during the operation of putting the nitrogen into the systems. Further eliminated is the need of having high-pressure booster pumps and banks of nitrogen bottles onboard the drilling rig. In addition, the required number of accumulators is reduced, since the useable fluid of the accumulator device of the present invention is greater than the conventional gas bag or bladder separator-type accumulator. Furthermore, by having fewer accumulator bottles on the BOP stack, the accessibility to other critical components is improved.
The present invention therefore allows for deployment of the BOP hydraulic equipment from the surface all the way down to the bottom of the ocean without having to stop and recharge, possibly risk contamination of the system with salt water or other debris, and improve on rig critical path efficiency. The features of the present invention are thus particularly advantageous with respect to deepwater drilling and production operations.